The influence of host immunologic responsiveness on the clinical course of transitional cell carcinoma is incompletely understood. Employing in vitro methods to study the lymphocyte function of individual patients in a prospective and longitudinal fashion it has been possible to observe certain patterns of test results which correlate with clinical course suggesting a predictive value. The following in vitro studies were performed: (1) Mixed Lymphocyte Culture (MLC), (examining the ability of the patients' lymphocytes to both "stimulate" and "respond"), and (2) Lymphocyte Blastogenesis induced by Concanavalin-A (Con-A). It was confirmed that the "stimulator" activity of some patients' lymphocytes was diminished with disease recurrence or progression suggesting it was predictive. Additionally, patient lymphocyte "Responsiveness" to Con-A and/or allogeneic lymphocytes was depressed by: (1) Radiation Therapy, (2) Tumor recurrence, and (3) Tumor progression. Tumor ablation by Surgery resulted in a return to normal of Con-A responsiveness in non-radiated patients and thus correlated with the clinical status of the patient. Radiated patients continued to manifest depressed (MLC) and Con-A responsiveness long after tumor ablation.